Frequently, large structures, such as offshore platforms, are anchored to the earth with tubular piles. Those piles are inserted through structural members of the platform and driven into the earth. They are then attached or "grouted" to the structural member. Often, these piles extend several hundred feet into the earth.
The length, number, and size of the pilings are in large part determined by the type of soil through which the piles are driven. Characteristics of the soil are normally determined before fabrication of the structure by analysis of soil samples or by other means. Unfortunately, in some cases, the soil characteristics are inaccurately predicted and it is found that the as-driven piles provide inadequate support for the structure after the structure is installed.
On other occasions, the soil qualities can be accurately determined, but it may be desirable to enhance the load capability of the piling. In still other cases, the structure is installed in permafrost and the frozen condition of the soil must be maintained to prevent settling of the structure.
Various methods of increasing or maintaining the load-bearing capacity of piles have been developed. For example, "anchor bumps" can be created on the pile to increase the load-bearing capacity and pull out resistance of the pile (U.S. Pat. No. 3,995,438). In some cases this may, however, not sufficiently increase the capacity of the pile.
Methods of maintaining the frozen condition of the soil have also been described (French Patent No. 475,226, see also U.S. Pat. No. 4,111,258). These methods rely on the circulation of cold ambient air through the pile to maintain the frozen condition of the soil. Such methods could not be applied in an area where extremely cold ambient conditions do not exist for a substantial portion of the year. Further, they provide only for the maintenance of the soil in the frozen condition to prevent subsidence and do not provide increased pull-out capacity.
Ground freezing has been used in order to provide temporary structural support while installing a subterranean tunnel, to prevent settling of a runway set on permafrost, and to prevent water encroachment during the installation of a ventilation shaft (Braun, B., and Nash, W. R., "Ground Freezing for Construction", Civil Engineering, January, 1985, pp. 54-56). In none of the above situations is a permanent method of substantially increasing the loadbearing and pull-out capacity of a tubular pile provided.
In summary, it is clear that an improved method of substantially enhancing the load-bearing and pull-out capacity of a pile is desirable.